Bias of CO2 surface fluxes estimated by eddy covariance due to "Adjustment fluxes"

The advection of horizontal inhomogeneous CO2 concentrations by a wind field with vertical gradient causes a height dependent vertical CO2 flux. It is shown that the corresponding bias between the flux at few meters above the surface and the flux through the surface is in the order of the natural CO2 flux variability through the air-sea interface, if horizontal CO2 gradients are assumed that are typical for land conditions or epicontinental seas. Although this bias is a zero mean effect on the long term, it is suspected that it may contribute significantly to the estimation uncertainty of air-sea CO2 transfer velocities based on eddy correlation flux measurements. A simple model is suggested to retrieve the surface flux from extrapolation of flux measurements at two height levels.